In particular with regard to third-generation (3G) wireless communication networks, Code Division Multiple Access (CDMA) techniques are used in the respective radio access network. In such a CDMA-based-wireless communication network and especially for the reverse link or uplink relating to the physical channel from user equipment, such as a mobile station, to the network, the effect of mutual interference is not negligible. Consequently the control of transmission power is very critical for the overall system performance of the radio access networks and hence, in CDMA-networks the power control is usually split into a fast inner loop and a slower outer loop of power control.
In detail, the inner loop providing fast power control has to mitigate changes in the ratio between signal and interference (SIR). Normally the changes of such signal to interference ratio are caused by fast fading, such as Rayleigh or Ricean fading, by shadowing, e.g. log-normal fading or by changes in the interference level. Ideally the received SIR should remain constant to enable a good reception of the reverse link signal without wasting transmit power at the user equipment. Actually, the inner loop has to adjust the transmit power at the user equipment such that the resulting SIR at the respective base transceiver station (BTS) of the network stays as close as possible to a target value of SIR.
The outer power control loop providing slow power control has to control the current link quality, usually in terms of bit error rate (BER) or block error rate (BLER), depending on requirements of the respective radio bearer service. The received link quality however may still change although the SIR is controlled by the inner power control loop. These changes are particularly caused by variations in a multipath delay profile based, for example, on typical urban and/or hilly terrain, by alterations in the speed of the user equipment or by modifications in the interference characteristics. Consequently, the outer power control loop has to adapt the aforementioned target value of SIR of the inner loop such that the required link quality is met.
In CDMA-systems according to the IS-95 standard, the uplink inner and outer power control loop, i.e. for the transmission from a user equipment to the network, are both located in the base transceiver stations (BTS). For the inner power control loop, the received SIR is estimated at the BTS and is compared against the target SIR. If the estimated SIR is greater than the target SIR, a power-down command is sent to the user equipment using the forward link control channel. Accordingly, a power-up command is sent if the estimated SIR is below the target SIR. Thus, the power command is generally based on values “up” and “down”. After the user equipment has received a respective power command, which is sent periodically 800 times per second, the transmit power is correspondingly changed by a predefined power step, usually based on dB. If the user equipment, however, is in a soft handover procedure, it combines the power commands of the respectively associated base transceiver stations. In particular the user equipment decreases its transmit power if any of the power commands asks the user equipment to decrease the transmit power and, hence, a transmit power increase is only performed if all power commands are power “up” commands.
With regard to the outer power loop control of such IS-95 systems, the BLER performance of a respective radio link is determined by checking the received frames or blocks for errors usually using a so called Cyclic Redundancy Check (CRC). If the CRC fails, the block is in error. To meet the required BLER, the target SIR is increased by predefined power up step in dB if a block was detected to be wrong. If the block was received correctly, the target SIR will be decreased by a fraction of the power up step. The correct BLER will be met if the power down step equals the power up step times the target BLER divided by “1-target BLER”. In soft handover procedures, however, all base transceiver stations execute their outer power control loop algorithms independently. Consequently, every base station tries to set the target SIR such that the respective required link quality is met.
Thus, one of the problems of a CDMA-system according to the IS-95 standard is that, in soft handover procedures, the uplink outer power control loop is only able to control the link quality of one respective link leg. Since all the links are combined in a frame selection means, which is located in a respective radio network controller, the outer power control loop is not able to control the overall link quality after frame selection is performed due to the fact that the outer power control loop is located in the base transceiver station where no information of the link quality after frame selection is available. Consequently, each of the link legs in soft handover procedures will try to achieve the target quality and, hence, the target SIR values at the base transceiver stations will be set higher than necessary. Thus, the SIR and, accordingly, the transmit power, will be higher than necessary, whereby such transmit power causes a waste in system capacity.
In a UMTS (Universal Mobile Telecommunication System) system the uplink inner power control loop is also located at the base transceiver stations, with the functionality of the uplink inner loop basically similar to the uplink inner power control loop according to the aforementioned IS-95 based system. The only difference is that the power commands are sent 1500 times per second.
However, different than IS-95 based systems, a system based on the UMTS is provided with an uplink outer power control loop which is located in the radio network controller and, thus, it is possible to evaluate the link quality immediately after frame selection is performed. Consequently, the link quality requirements are related exactly to the measurable link quality.
One of the main drawbacks of such a UMTS decentralized power control concept, however, is the large signaling delay between the base transceiver stations and the network controller. In particular, with respect to the power control, the signaling delay located between the inner and the outer power control loop is degrading the speed and performance of the outer power control loop significantly. This causes a reaction to changes in the environment to be delayed, and the control loop needs to slow down to avoid instability problems. Moreover, the decentralized architecture also implies an additional signaling traffic load on the link between the radio network controller (RNC) and an associated NodeB, which may result in problems for radio network operators who depend on third party operators for the connection between the radio network controllers and the NodeBs.
Accordingly, an object of the invention is, to provide with regard to the aforediscussed state of the art, a new and significantly improved approach for controlling the transmission power in the uplink direction of a CDMA-based radio system avoiding the current drawbacks or problems and, in particular, providing a transmission power control even with regard to an overall link quality substantially involving a real time reaction on changes in the environment.